A knee joint includes a femur bone, a tibial bone, and a patella, which are replaced by artificial materials such as ultra-high-molecular polyethylene or titanium alloy when an osteoarthritis of the knee, etc. is contracted. These replacing parts are referred to as a femur bone component, a tibial bone component, a patella component, or the like. A patella may not be replaced by the component depending on doctor's diagnosis.
A load, which is several times larger than body weight, is exerted on a knee joint when walking, etc. Accordingly, if each of the components constituting the artificial knee joint is not properly aligned, the load concentration on the specific parts, etc. may cause problems such as shortening the life of artificial knee joint and preventing a smooth walking.
As such, a quantitative assessment of therapeutic effect may be made if it is possible to recognize and assess after an artificial joint replacement that the artificial joint replacement has been made with a proper alignment. Generally, medical appliance embedded in a living body is formed in a smooth shape without an edge point or the like so as to mitigate an aggressive character against a living tissue. This is true for a hard tissue such as bone in a living body, too.
X-ray photography is practically used for the assessment. It is known that accuracy is improved by using an image analysis, etc. without making visual evaluations on X-ray images.
However, an artificial joint embedded in a living body and surrounding bones being formed with smoothly curved surfaces as described above, it is not easy to estimate their position and attitude by recognition of change in locations of the identified feature points.
Non-patent literature 1 shows a simple and a high-accuracy method, which enables to estimate a 3D position and attitude based on a single X-ray image by pattern matching using Fourier descriptors. The pattern matching using Fourier descriptors is used in various fields. That is, the contour, which is made by projecting an object onto a 2D plane, is divided by nodes. And, the numerical sequence made by coordinates of the nodes is compared to that of the contours that are preliminarily photographed under various conditions. The contour of the object, which coincides with the condition, is found by mathematical procedures, and thus the condition of the object is estimated. As such, the pattern matching using Fourier descriptors is practically used as a method of estimating 3D position and attitude of an object including smoothly curved surfaces such as medical appliance embedded in a living body or surrounding hard tissues. Fourier descriptors, especially those using closed curved lines, can be conveniently used for comparison regardless of size, position of the center of gravity or direction of the obtained contour during the process of normalization.
Prior art is described hereinafter.
Patent literature 1 shows a method of estimating a distance between an X-ray source and the photographed artificial joint of a patient with reference to contours of X-ray images preliminarily photographed in various distances, making use of the fact that the contour of artificial joint in X-ray 2D image is varied depending on the distance between an X-ray source and an artificial joint. According to this method, the distance between the X-ray source and the artificial joint may be estimated, but the attitude of the artificial joint cannot be estimated. Further, since the overall contour of the photographed object (artificial joint) is used, the distance cannot be estimated if a part of the contour is deficient or unclear.
Patent literature 2 shows an image processing device in which a matching is performed between 3D shape data of bone and 2D X-ray images. Feature points called “bone's axis” are manually defined in 3D shape data of bone to produce their 2D projection images. Then, a projection location is determined such that displacement between the locations of the above images and those of feature points in 2D X-ray images is reduced to minimum, and thus the location of bone, etc. can be estimated. Since it includes a calculation part of virtual projection images, projecting 3D shape data onto 2D plane, calculation is required each time the location is estimated. However, the computing speed is limited. Further, since bone's axis is defined manually, it is difficult to define bone's axis for an object including smoothly curved surfaces without projections, etc., and thus the location estimation cannot be performed.
Patent literature 3 shows a method of retrieving similar 3D models based on 2D sketch images by preparing a contour database for 3D models. Fourier descriptors are used for the above retrieval to express the 2D sketch images and the data of the contour database for 3D models. Since the sketch images are supposed to be expressed by closed curves, if a small gap exists due to hand-written lines, a closing preprocessing is performed to fill the gap and make closed curved lines before the retrieval. As such, if a part of the contour of the object is deficient or unclear, the estimation is impossible. Furthermore, since the retrieval is the subject matter of this reference, the estimation of the 3D position and attitude cannot be performed.
Patent literature 4 shows a method of recognizing a pattern by using a neural network even when a part of the contour of 2D images is deficient. However, since this reference does not include a database for contours, a database matching for contours, which is required to classify input image patterns into one of preliminarily registered categories for discrimination of the position and attitude, cannot be performed. And thus, the estimation of the position and attitude cannot be performed.
Non-patent literature 1 shows a method of estimating a position and attitude of an artificial joint based on X-ray images using closed curve Fourier descriptors. Also, non-patent literature 2 shows an example using a similar method. In these technologies, numerical sequences for contours of an object to be photographed with variously different attitudes are preliminarily prepared and accumulated in a database in the form of closed curve Fourier descriptors. Contours of an object photographed in any attitude, which are expressed in the form of closed curve Fourier descriptors, are compared to data stored in a database, and thereby contours on the database, with the least errors, are selected. As such, an angle of the object photographed in any attitude is recognized (the same attitude as the selected contour on the database is recognized). Since nothing but comparison is implemented with numerical sequence data preliminarily stored in the database, 3D position and attitude can be advantageously estimated in a short calculation time. However, these technologies employing closed curve Fourier descriptors, the distance cannot be estimated if a part of the contour of a photographed object is deficient or unclear.